High ambient illumination imposes severe problems on an EL display panel inasmuch as legibility of the information being displayed is reduced under such conditions. The cause of this poor legibility is the reflected ambient light from the back metal electrodes and other interfaces in the display panel which reduces the contrast ratio of the lit to the unlit pixels on the panel, i.e., the ratio of the brightness of the displayed image to the brightness of the background of the panel.
Of the several solutions that have been proposed to solve this problem, the one that provides for negligible internal reflection and diffusion of the ambient light is the placement of a light sink layer immediately behind the phosphor layer. A material which is admirably suited for such a light sink layer is disclosed in a copending U.S. patent application, Ser. No. 541,702, filed Oct. 13, 1983, by Ramachandra M. P. Panicker, and assigned to the same assignee as the present invention. This material is a light absorbing semiconductor compound which is especially adapted to have a high specific resistivity.
In particular, this light sink layer material is characterized as having a high light absorption coefficient and an index of refraction which is much higher than the two adjoining layers, namely, the phosphor layer on the front thereof and the dielectric layer on the rear thereof. Such a light sink layer results in the ambient light incident thereon not being reflected but, rather, trapped by total internal reflection and absorbed within the layer. Moreover, the performance of a matrix-addressed EL panel is greatly improved by the use of such a high resistivity light sink layer material in that the voltage drop thereacross is reduced and the steepness of the luminance vs. voltage characteristic curve is increased as desired for multiplexing operation of such an EL panel.
As pointed out in the above referred to patent application, although the light sink layer made of a semiconductor material having a high specific resistivity, on the order of 10.sup.8 to 10.sup.12 ohm-centimeter, was closer to being a dielectric, nevertheless, during the refreshing of a pixel on the panel by the use of an alternating drive voltage, it was only during the half cycle that the back electrode was positive in potential that light was generated in the panel. A negligible amount or no light was generated during the half cycle that the front electrode was positive in potential.
However, in spite of the fact that the EL panel only generated light in the phosphor layer during the half cycle that the back electrode was positive in potential, nevertheless, the fact that incident ambient light impinging on the light sink layer was not reflected but totally absorbed therein not only caused the background of the panel to be black but so greatly increased the contrast ratio of the panel that it was readable under sunlight conditions, for example. It should be appreciated, however, that in view of the need to be able to read panels under all kinds of severe ambient lighting conditions and the continued effort to improve the efficiency of the panel, it is highly desirable to still further increase the light output and, therefore, the contrast ratio of the EL panel.